The invention pertains to radio frequency (RF) wireless data transmission.
There are many different techniques and protocols for modulating data onto carrier frequencies for radio transmission. With the ever-increasing number of wireless transmitters and receivers sharing the limited available bandwidth, radio system manufacturers are always looking for techniques to increase data transmission rates so as to maximize the numbers of communication channels available within a frequency band and to maximize the baud rate of each channel.
For instance, GMSK (Gaussian Filtered Minimum Shift Keying) is the modulation technique used in GSM (Global System for Mobile Communications). It has many advantages, including spectral efficiency and constant envelope modulation. That is, the amplitude of the signal is essentially fixed, such that it has a zero Peak-to-Average Power Ratio (PAR). Accordingly, it is particularly suitable to mobile applications such as cellular telephones because the amplifiers for radios used in such systems need to operate efficiently only within a very narrow power range and can therefore be made small and lightweight. However, one significant disadvantage of GMSK is that the baud rate must be much lower than the Nyquist rate.
QAM (Quadrature Amplitude Modulation) is another modulation technique used in many radio systems. QAM has the advantages of being able to support a baud rate close to the Nyquist rate. However, it has the disadvantage of being a phase and amplitude modulation technique. Therefore, it has a high PAR, thus requiring the use of more sophisticated amplifiers that can operate efficiently over a relatively broad amplitude range. Multi-carrier QAM modulation techniques, such as OFDM, for instance, have a PAR in the range of 10 to 1 or greater. This generally requires an amplifier to be bulkier and, therefore, disadvantageous particularly in connection with mobile applications.
In LMR, most older systems tend to employ constant envelope modulation techniques because of its power efficiency. But these techniques are not bandwidth efficient. Therefore, newer LMR systems tend to use non-constant envelope waveforms such as QAM or multicarrier waveforms of various types.
Conventional single-carrier QAM systems tend to suffer from ISI (Inter-Symbol Interference) resulting from propagation channel factors such as Doppler shift, multipath interference, and distortion. These systems typically utilize adaptive equalizers in the receiver to compensate for the effects of the propagation channel. The non-stationary nature of the propagation channel, particularly in mobile applications such as police car radios or personal radio systems worn by persons who are running or riding in a car, has proven challenging.
Multi-carrier modulation techniques have been introduced to divide the propagation channel into smaller segments so that channel impairments can be handled discretely in narrower bands and treated as constant across these narrower bands. These narrow segments are less complex to equalize using frequency domain equalization. However, multi-carrier waveforms have high PAR. A radio operating in a system with an exemplary PAR of 10 dB must be capable of generating signals having amplitudes that are ten times the average power of the waveform. Accordingly, they require higher power amplifiers that can be physically large.
Recently, a new type of modulation technique known as SC-FDMA (Single Carrier-Frequency Division Multiple Access) has shown promise. See for instance: Hyung G. Myung, “Single Carrier FDMA for Uplink Wireless Transmission,” IEEE Vehicular Technology Magazine, September 2006. SC-FDMA is an OFDM (Orthogonal Frequency Division Multiplexing) technique. OFDM is a technique suitable for broadband data communication. Since its original introductory in the 1960's, OFDM has been widely adopted for broadband wireless communication systems, such as WiFi and WiMAX. The key concept of OFDM is multi-carrier modulation. The sub-carriers are orthogonal to each other for maximum efficiency. The orthogonal multi-carrier modulation can be performed efficiently by using an IFFT (Inverse Fast Fourier Transform) algorithm. The bandwidth or spacing of these sub-carriers is small enough that the RF channel can be considered constant. Hence, a simple channel equalization scheme can be used. OFDM symbols often includes a CP (cyclical prefix) to absorb ISI (inter symbol interference). The weakness of conventional OFDM is the high PAR (peak to average ratio). The OFDM waveform has a PAR around 10 dB.
On the other hand, SC-FDMA, which is also called DFTS-OFDM (Discrete Fourier Transform Spread OFDM), has relatively lower PAR than OFDM and is easily equalized in wide band applications, such as cellular telephone applications, where channel bandwidths are measured in MHz. Compared to conventional OFDM, SC-FDMA adds a DFT spreading block before the IFFT when generating the waveform.
The sub-carrier spacing and CP length are key to implementing OFDM or SC-FDMA in wideband applications to minimize PAR and ISI. Normally, these two parameters are set as a function of coherent channel bandwidth and delay spread. Sub-carrier spacing typically is set to be roughly equal to coherent bandwidth, and CP is set large enough to absorb delay spread. In both cases, the sub-carrier spacing is based on the coherent channel bandwidth, and the CP length is based on delay spread.
However, these implementation techniques do not apply to narrowband wireless communication. For example, in a narrowband wireless channel of 25 KHz, a normal OFDM or SC-FDMA waveform would have only two sub-carriers. Such a waveform cannot meet a typical spectrum mask or ACP requirement for such a channel. However, in narrow band radio systems, such as many LMR systems, where the channel bandwidth typically is about 12.5 KHz or 25 KHz, or about 1/50th the channel bandwidth of typical cellular telephone systems, it is much more difficult to apply the conventional SC-FDMA techniques for minimizing ISI and PAR.